1. Technical Field
The present invention relates generally to a mobile communication system using, for example, TDMA (Time-Division Multiple-Access) or FDMA (Frequency-Division Multiple-Access) technique, and more particularly to an improved antenna arrangement for use in cellular radiotelephone communication systems which is designed to reduce the co-channel interference for shortening the distance between radio zones or cells to which the same frequency is assigned for improvement of the spectrum efficiency.
2. Background of Related Art
In conventional cellular radiotelephone communication systems, an omnidirectional antenna or several directional antennas are disposed in a base station to define a circular radio zone or cell. In a TDMA or FDMA system, transmission of a plurality of radio signals at the same frequency will induce the co-channel interference, resulting in a failure in data transmission.
FIG. 21 shows one example of conventional zone layout. Each hexagonal cell has disposed centrally a base station. A number labeled in each cell indicates the frequency. The cells of the same number are assigned the same frequency. In this frequency reuse pattern, reduction in co-channel interference is achieved by increasing the distance between the cells to which the same frequency is assigned. This, however, decreases the spectrum efficiency, thereby resulting in a difficulty in increasing the capacity of telephone subscribers.
In order to avoid the above problem, Japanese Patent Second Publication No. 3-37336 teaches a cellular telecommunication system using the sector layout as shown in FIG. 22.
A cell is divided into six sectors 1 to 6 surrounding a base station 7. Six directional antennas each covering a fan-shaped part of the cell are disposed on the base station. An increase in frequency reuse is achieved without lowering the DU (desired signal to undesired signal) ratio by assigning the same frequency group to any two of the antennas oriented in opposite directions. For example, the same frequency group F1 is assigned to the sectors 1 and 4. The same frequency group F2 is assigned to the sectors 2 and 5. The same frequency group F3 is assigned to the sectors 3 and 6. This frequency reuse will cause, for example, the sector 1 to experience the co-channel interference with a radio signal from the sector 4 which uses the same frequency channel and is diametrically opposed to the sector 1. A radio signal radiated from the sector 1 to sector 4, however, has a level that is decreased by a front-to-back (F/B) ratio (i.e., ratio of signal strength transmitted in a forward direction to that transmitted in a backward direction). Typical antennas show a F/B ratio of 25 dB or more.
The above prior art communication system having the antennas disposed at the center of the cell, however, encounters the difficulty in improving the spectrum efficiency for the following reasons.